1. Field of the Invention
The present invention relates to a cutter for an earth boring drill bit and more particularly to such a cutter comprising a central stud adapted for mounting on the bit body and having a layer of abrasive material, such as polycrystalline diamond, formed about the circumference thereof.
2. Description of the Related Art
One type of earth-boring drag bit includes a bit body having a plurality of cylindrical studs extending from the crown thereof. Each of the studs includes a polycrystalline diamond (PCD) cutting element mounted thereon. The cutting element is typically in the shape of a cylindrical disk having one side thereof bonded to the stud. The cutting elements are oriented so that when the bits are set on the bottom of a borehole an edge of each cutting element, and a portion of the stud upon which the cutting element is mounted, abuts against the formation. As the bit rotates, the stud wears away more rapidly than the PCD cutting element thereby forming a cutting element lip which extends beyond the stud. It is the edge of this lip which cuts rock from the formation as the bit rotates.
When drilling in hard and sometimes abrasive formations, the stud may wear rapidly thus leaving a relatively large lip which breaks off as a result of the forces applied to it during drilling. Although it is normal for the outermost portion of the lip to break off from time to time as the stud wears away, if this occurs too frequently wear is very high and the rate of penetration may be reduced. It would be desirable to provide a cutting element on a drill bit in which lip size is limited even in hard or abrasive formations. It would also be desirable to design a lip depth for a bit used in a particular formation.
The usual drill bit of the type above described rotates about a central axis. The cutters on such a bit therefore cut in only a single direction, namely the direction of cutter movement during bit rotation. Some bits, however, require cutters which can cut from more than a single direction. For example, some bits are constructed to create a bore which turns rather than simply having a straight axis. Such bits can be navigated to create a curved bore. When such a bit turns, the cutters thereon cut in a different direction than when the bit moves along a straight axis. It would be desirable to provide such a drill bit with a cutter which could cut in any direction.
Another problem inherent in using a conventional drill bit to drill hard formations is that the center of the axis of rotation the velocity of the bit is very low. It would be desirable to provide a drilling system which provided relatively uniform cutter velocity across the entire bottom surface of the borehole.